A combined Sm–Nd, Rb–Sr, and U–Pb isotopic study of Mg-suite norite 78238: Further evidence for early differentiation of the Moon

Abstract

Lunar Mg-suite norite 78238 was dated using the Sm–Nd, Rb–Sr, and U–Pb isotopic systems in order to constrain the age of lunar magma ocean solidification and the beginning of Mg-suite magmatism, as well as to provide a direct comparison between the three isotopic systems. The Sm–Nd isotopic system yields a crystallization age for 78238 of 4334 ± 37 Ma and an initial ε Nd 143 value of −0.27 ± 0.74. The age-initial ε Nd 143 (T-I) systematics of a variety of KREEP-rich samples, including 78238 and other Mg-suite rocks, KREEP basalts, and olivine cumulate NWA 773, suggest that lunar differentiation was completed by 4492 ± 61 Ma assuming a Chondritic Uniform Reservoir bulk composition for the Moon. The Rb–Sr isotopic systematics of 78238 were disturbed by post-crystallization processes. Nevertheless, selected data points yield two Rb–Sr isochrons. One is concordant with the Sm–Nd crystallization age, 4366 ± 53 Ma. The other is 4003 ± 95 Ma and is concordant with an Ar–Ar age for 78236. The 207Pb– 206Pb age of 4333 ± 59 Ma is concordant with the Sm–Nd age. The U–Pb isotopic systematics of 78238 yield linear arrays equivalent to younger ages than the Pb–Pb system, and may reflect fractionation of U and Pb during sample handling. Despite the disturbed nature of the U–Pb systems, a time-averaged μ ( 238U/ 204Pb) value of the source can be estimated at 27 ± 30 from the Pb–Pb isotopic systematics. Because KREEP-rich samples are likely to be derived from source regions with the highest U/Pb ratios, the relatively low μ value calculated for the 78238 source suggests the bulk Moon does not have an exceedingly high μ value.